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Write Call

What you will see is the write line (Listing 10). In both the Perl and the C cases, what happens eventually is that a write call is made to output a string (the second argument in the method call) to the screen. In difference the case of Perl is that this is done via the interpreter, whereas in the case of C, it's the program itself that makes the call (but you can't tell this from the line, just from the context of it.)

Listing 10

Write Line

01 write(1, "Hello World", 11) = 11

Once again, there's an exit_group call, but this time it's called on the write call (value 11, in my example), rather than on the interpreter (value 0), as in the Perl case.

Conclusion

This quick introduction shows some basic information that you can learn from studying system calls with strace. Also, you might want to try strace out on a "Hello World" Bash script, or you could also try running strace on short commands such as cd or pwd.

The Author

Juliet Kemp has been playing around with Linux ever since she found out that it was more fun than Finals revision and has been a sys admin for about five years. She finds strace output deeply fascinating, and had great fun delving into system call man pages while researching this article.

Linux lets users watch the kernel at work with a little help from Ptrace, a tool that both debuggers and malicious process kidnappers use. A CPAN module introduces this technology to Perl and, if this is not enough, C extensions add functionality.